THE president's ADDRESS; UT 



parasitic life and establishing themselves in the body of their 

 host, overcoming all resistance to their invasion. This kind of 

 newness was probably not accompanied with lethal powers on the 

 part of the parasite, since the capacity for reciprocal attack and' 

 defence on the part of the two organisms concerned was pro- 

 bably developed gradually in each organism and would tend to 

 balance each other. The newness which is dangerous is seen 

 where an organism with fully developed powers of parasitism in 

 one " tolerant " host acquires the capacity for living in another 

 host not adapted to it. This consideration throws perhaps some 

 light on the disease-producing powers of trypanosomes in general. 

 From the known facts of the development of trypanosomes, it 

 seems highly probable that they have in all cases two hosts, a verte- 

 brate and an invertebrate. There are further many considerations 

 which indicate that they were originally parasites of the digestive 

 tract of the invertebrate host, and that when the latter took 

 to blood-sucking habits, the trypanosomes became accustomed to 

 nourishing themselves on blood, and were finally able to establish 

 themselves in the vertebrate host. This view may explain why 

 these flagellates are always perfectly harmless to the invertebrates, 

 their "old" hosts, and only pathogenic to vertebrates, their "new" 

 hosts. 



Returning now to the consideration of the examples specially 

 selected, it is seen that the harmlessness of the leivisi-group is 

 due to the fact that the species are specifically adapted to certain 

 hosts and cannot maintain themselves in anv other but those 

 particular hosts. On the other hand, the lethal powers of the 

 5nfcu-group are associated with the fact that its members, even 

 when specifically adapted to a " natural " host, can nevertheless 

 establish themselves also in a wide range of " new " hosts, with 

 fatal results to the latter. If I am right in regarding the 

 leivisi- groM'p as further advanced in specific differentiation than 

 the brucii-group, it then becomes probable that the natural ten- 

 dency of evolution is to bring about a balance in the profit; and loss 

 account of parasite and host, and that the species of pathogenic 

 trypanosomes of the Srztm-group would tend in the future to 

 become harmless parasites of certain hosts. This evolution, how- 



JouRN. Q. M. C, Series II. No. QQ. 2 



